Subsystems comprising disk arrays, i.e., groups of small, independent disk drive modules used to store large quantities of data have been developed and found to possess many advantages over a single large disk drive. For example, the individual modules of a disk array typically take up very little space and typically use less power and cost less than a single large disk drive, yet, when grouped together in an array, provide the same data storage capacity as a single large disk drive. In addition, the small disks of an array retrieve data more quickly than does a single large disk drive because, with a small disk drive, there is less distance for the actuator to travel and less data per individual disk to search through. The greatest advantage to small disk drives, however, is the boost they give to I/O performance when configured as a disk array subsystem.
A disk array is typically housed in a chassis, which may be an enclosure, for holding a plurality of disk drives. When additional memory storage space is required, it is desirable to be able to add an additional disk array chassis to the existing system. It is further desirable that one be able to add the new disk array chassis to the existing system without needing to shut down the existing system. In other words, it is desirable to provide a dynamically upgradeable disk array system.
For a system including a disk controller communicating over a loop with a plurality of disks, it is undesirable to expand that bus by simply adding more disk drives. There may be a malfunction in any of the new disk drives. Further there may be a problem with the new connection to the existing loop. While the functional status of the new equipment can be tested by the disk controller after it is connected to the loop, if there is a malfunction, the entire loop (and disk drives) will be degraded and may be rendered inoperable or inaccessible. Therefore, it is desirable to be able to determine the integrity of the new disk drives and bus before permitting interface with the existing system.